The present invention relates generally to systems and methods for measuring velocity, and more particularly, to methods and systems for Doppler velocimetry providing directional vectors.
Laser Doppler velocimetry (LDV) has been in development for several decades and provides one of the best methods for high-accuracy, single-point, fluid or surface velocity measurement. However, typical LDV systems and methods cannot easily determine a direction of the fluid flow. This has been addressed in various ways. By way of example, a Bragg cell can be used in a two-beam laser system to create a frequency shift between the laser beams.
One of the earliest methods of making the LDV system direction-sensitive used a mechanically rotating diffraction grating in the optical path. FIG. 1 is a schematic of a typical LDV system 100 with a mechanically rotating diffraction grating 106. Typically, the rotating diffraction grating 106 is produced by mounting the grating radially on a small wheel mounted coupled to a motor 108. The motor 108 then rotates the grating continuously.
A laser source 102 emits a laser beam 104 along a light path 105 toward the mechanically rotating diffraction grating 106. The mechanically rotating diffraction grating 106 diffracts the laser beam 104 into two laser beams 104A and 104B. It should be noted that for clarity purposes only the +/−first diffraction order laser beams 104A and 104B are shown in FIG. 1 and that higher diffraction order laser beams (not shown) can also be produced.
The mechanically rotating diffraction grating 106 is typically moved by a motor 108. The mechanically rotating diffraction grating 106 moves transverse to the laser beam 104 with a velocity v so that the first order diffracted laser beams 104A and 104B are shifted up and down in frequency, respectively. The diffracted laser beams 104A and 104B are then incident on an input surface 110A of the lens 110. The lens 110 focuses diffracted laser beams 104A and 104B. The lens 110 emits the focused diffracted laser beams 104A′ and 104B′ from an output surface 110B toward a measurement volume 112. Where the focused, diffracted laser beams 104A′ and 104B′ cross is the measurement volume 112. Interference fringes are formed in the measurement volume 112. Due to the motion of the diffraction grating 106, the fringes 112A-112D in the measurement volume move in the y direction with a frequency 2v/d.
The diffraction grating 106 performs two roles in the above system 100. First, rotating the diffraction grating 106 splits the input laser beam 104 into two laser beams 104A and 104B. If a sinusoidal phase diffraction grating 106 is used, only two laser beams 104A and 104B may be generated. If a non-sinusoidal diffraction grating is used all but the +/−1 first diffraction order laser beams are blocked.
Secondly, mechanically moving the diffraction grating 106 induces a frequency shift onto the two diffracted laser beams 104A and 104B. One diffracted laser beam 104A shifts up in frequency and the other diffraction order laser beam 104B shifts down in frequency. This frequency shift causes the fringes to move.
The two, focused diffracted, laser beams 104A′ and 104B′ recombine in the measurement volume 112. When a particle 120 is stationary in the measurement volume there is a fluctuation of the scattered light intensity as the interference fringes move past the particle. The frequency of these fluctuations is 2v/d. Light is reflected from the particle in all directions (i.e., scattered).
When the particle moves it will either move in a direction opposite of the moving fringes (i.e., forward) or in the same direction as the moving fringes (i.e., reverse). When the particle moves forward, the measured frequency of the intensity fluctuations is more than 2v/d. When the particle moves in reverse, the measured frequency of the intensity fluctuations will be less than 2v/d. This increase or decrease of the frequency allows the direction of motion to be determined.
The mechanical rotating grating system 100 of frequency shifting works and is commercially available. The mechanical rotating grating system 100 has several shortcomings. One shortcoming is a spatially varying fringe 112A-112B period in the measurement volume.
In another shortcoming, the motor 108 induces vibration into the mechanically moving grating 106. This vibration causes additional intensity fluctuations.
In view of the foregoing, there is a need for an improved LDV system capable of determining direction and yet without needing a rotating grating.